fix(grip): stop double-offsetting the UCS grip snap marker

The extension anchors needed the same pane-to-canvas lift the snap
marker already got, but the block that added it was placed next to the
one that was already there rather than merged into it, so `screen` took
the tile origin twice. A single full-canvas tile has no origin to add, so
it only shows once the viewport is split. Fold the anchors into the
existing block.

Also require the ray to pass through the snapped point before offering it
as a distance reference. Only four acquisitions are kept and the oldest is
dropped as new ones arrive, so the ray that produced a snap can be gone by
the time a distance is typed; picking the nearest survivor unconditionally
sent the point off along an unrelated ray instead of falling back to plain
point input. `extension_snap` projects onto the ray in exact f64, so the
tolerance only has to clear rounding, scaled with the coordinate to hold at
UTM magnitudes.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/app/update/viewport.rs

@@ -2043,24 +2043,19 @@ impl OpenCADStudio {
         let world = snap_hit.map(|s| s.world).unwrap_or(raw);
         self.tabs[i].snap_result = snap_hit;
         if let Some(s) = self.tabs[i].snap_result.as_mut() {
+            // Snap marker and its extension anchors are pane-local; lift them to
+            // absolute canvas px.
             s.screen.x += tile_b.x;
             s.screen.y += tile_b.y;
-
             if let Some(base) = s.extension_base.as_mut() {
                 base.x += tile_b.x;
                 base.y += tile_b.y;
             }
-
             if let Some(base) = s.extension_base2.as_mut() {
                 base.x += tile_b.x;
                 base.y += tile_b.y;
             }
         }
-        if let Some(s) = self.tabs[i].snap_result.as_mut() {
-            // Snap marker is pane-local; lift it to absolute canvas px.
-            s.screen.x += tile_b.x;
-            s.screen.y += tile_b.y;
-        }
 
         use acadrust::types::Vector3;
         let v3 = |d: glam::DVec3| Vector3::new(d.x, d.y, d.z);